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Risultati sfruttabili
Controlled deposition of magnetic qubits in pure quantum spin states:
- Approach 1: In our group we have developed a new soft, reliable and simple methodology to address individual Mn12 molecules onto a film surface. Such a methodology is based on the preparation of nanocomposite thin films, made from a polycarbonate polymeric matrix and an Mn12 complex, and the subsequent treatment with different organic solvent vapours. Solvent treatment originates that a small fraction of the Mn12 molecules emerge to the surface of the film with an aggregation state that can be controlled at will, depending on the nature of the solvent. Moreover, we have shown that the location Such a methodology is based on the preparation of a polymeric thin-film made from a polycarbonate matrix and Mn12 molecules, which is moulded with a master and exposed to vapours of an organic solvent. Such a solvent treatment smooth out the surfaces relieves of the polymeric replica and the Mn12 molecules that were dispersed in the protruding regions of the replica concentrate in the original protruding features but do not diffuse laterally thus originating a compositional contrast of Mn12 on the polymeric surface. Magnetic response of the patterned aggregates of Mn12 molecules, using Magnetic Force Microscopy (MFM), follows accurately the compositional spatial modulation of the pattern, so it is possible to encode and read out information with such molecular patterns working at the paramagnetic regime from mesoscopic down to nanometre length scales. This is the first time that small aggregates and individual SMM's are deposited and can be individually screened either by AFM and MFM microscopies. The advantages of this approach are considerable. First, the polymeric matrix plays a critical role in stabilising the SMM as well as enhancing the overall mechanical strength of the film. Second, polycarbonates are a commercially important and technologically interesting polymers that have a combination of properties not found in any other plastic, including very high impact strength, creep resistance, optical clarity, and a low moisture absorption. And finally, the resulting nanocomposite thin films are fully compatible with nowadays magnetic and magneto-optical storage technologies, where polycarbonate resins are used as the basic support for the fabrication of disks.
- Approach 2: Along the second year, in collaboration with an Italian team, we have demonstrated that arrays of nanometre-sized aggregates, each made of a few hundred single-molecule magnets derived from Mn12 complexes, can be patterned on large areas by self-organisation assisted by stamps on a surface in a de-wetting regime. The large length scale is imposed by the motif of the stamp protrusions, and the smaller length scales, viz., the size and distance of the molecular aggregates, are controlled by deposition and growth phenomena occurring in a volume confined beneath the protrusions by capillary forces. The method is general to a variety of molecular materials and substrates because repulsive, as opposed to specific, interactions are required. Our result hints at the possibility of sustainable patterning of single-molecule magnets for ultra-high-density magnetic storage and quantum computing.
Mn12 patterned arrays supplied by CSIC were imaged by NPL using scanned probes ranging from AFM, MFM STM to UHV-Low temperature STM. The imaging of magnetic nanoparticles by scanned probes as we demonstrated is also a vital aspect for many of the areas of emerging nano-technology; for example, this work impacts on data storage devices and new magnetic storage medium based on magnetic nano particles. The manipulation of nano-particles needed for qubit preparation is also applicable to a wide variety of applications where a bottom-up device fabrication route is needed.
The work of NPL in this area also lead to collaborations and interactions with the University of Posrtsmouth where we used the techniques learnt in this project to image and manipulate magnetic beads attached to DNA. An aspect that arose is that the measurement of magnetism on a nanoscale is both qualitative and non-traceable at the present. If future claims on magnetic properties of nanoparticles is to be verified, then work is needed to develop methods and standards that can address this problem. NPL and its European partners are well placed to take up this challenge.
On the other hand, as the project developed, interest shifted into microwave superradiance emission and detection in magnetic assemblies along the third year, and CSIC-IMM was asked to stop in-house development of more promising small magnetic moment detection by nano-electro-mechanical devices fabricated on GaAs and to supply tools for microwave detection and surface acoustical wave (SAW) devices.
NPL measured and investigated SQUID based devices. These ranged from Nb devices to Al devices and in size from a few microns down to 200nm loop dimensions. Sensitivity and applicability to Mn12 spin detection were addressed.
A novel method of using a SQUID to detect its own sensitivity using a magnetic tipped STM system was developed. This impacts in several areas as it shows the ability to use a magnetic tipped STM to achieve magnetic information on a very minute length scale and also shows the ability to combine two quantum detection systems, an STM and a SQUID in one instrument allowing a combination of topography and magnetic information to be obtained simultaneously. This has potential to impact on several technologically significant areas from magnetic storage to quantum computation.
Development of SAW interdigitated transducers on LiNbO2. Technology for surface acoustical waves (SAW) transducers will impact also on future measurement techniques and applications in the biosensor field.
A strong work on the interaction between microwave radiation and molecular magnets was performed by the UB group. Efforts focused on two main problems:
- First, the analysis of the electromagnetic radiation accompanying the fast demagnetisation process in molecular magnets and,
- Second, performing new experiments to better understand the spin tunnelling transitions in the case of extremely high sweep of the bias magnetic field.
In fact, these two works connected the behaviour of our magnetic units as qubits and the superradiance emission.
The experiments to better analyse the electromagnetic radiation coming from molecular magnets were performed using:
a) different molecular magnets to correlate the chemical and nuclear properties of the magnetic unit with the probability of emission of radiation,
b) under different experimental conditions to study the influence of the temperature, magnetic field and number of molecules on the power emission of the electromagnetic radiation,
c) using different waveguides to better correlate the power transmission with the geometry, diameter, length and material, of the waveguides, and
d) using very fast detection methods for the magnetization change and radiation detection.
The main aim of the experiments performed at ultra high sweep of the bias magnetic field was to test the Landau Zener transition probability in the spin tunnelling case. It is well established that single quantum transitions obey the Landau Zener law. In our experiments, however, we have seen that this law is not obeyed anymore and this has been interpreted as due to the simultaneous transition of a huge number of spin levels. This result provides strong evidence for the occurrence of the superradiance emission. These experiments are also crucial to better understand the qubit behaviour of the magnetic clusters when working as logic gates.
Reversal of the Mn12 cluster spin can occur either classically, by thermal activation, or quantum mechanically, by tunnelling through the barrier. SMMs are attractive model systems to study the effects of coupling magnetic qubits to the environment (nuclear moments, phonons), with the associated problems of decoherence and the limits of quantum mechanics at the large scale. For both aspects the hyperfine coupling between cluster spin and nearby nuclear spins is expected to play a crucial but subtle role: since this coupling is many orders larger than the quantum tunnelling splitting, a static hyperfine interaction completely blocks tunnelling. To get a better insight into these mechanisms, single crystals of a deuterated sample of the well-known Mn12Ac ([Mn12O12(CD3COO)16(D2O)4].2CD3COOD.4H2O) were obtained following the methodology reported in the literature by Lis for the synthesis of Mn12Ac by using deuterated acetic acid and D2O.
Concerning the development of SQUIDs as potential Mn12 based Qbit detectors, NPL has tested many and developed several devices. Devices were obtained from commercial fabrication facilities, HYPRES in US, from world leading research groups, s Wernsdorfer in Grenoble and Lam, CSIRO in Australia. Also both Aluminium and Niobium based devices from CSIC-IMM were tested as well. We also sent some of our devices to CSIC-UB for inclusion in their experiments on microwave emission from Mn12 single crystals. Sensitivity of the devices was measured as well as a full characterisation of their I-V curves as functions of temperature and their response to magnetic fields. Using a newly acquired apparatus for this project, an Adiabatic demagnetisation refrigerator, we were able to test devices down to 50mK simulating the operating conditions of the devices if they are to be used as QBit detectors. Using our measurements we were able to give feedback to the fabrication process so that better devices could be made. An example was the recommendation that a Gold overlayer of specific thickness be applied to the Niobium devices from CSIRO and that the geometry of the junction design was changed. This work has enabled us to form relations and synergies with these important groups and so to raise our profile and capability within Europe. These devices have potentially a wide range of applications that can impinge on various aspects of the quality of life, for example as THZ detectors for security screening.
Objectives of the CSIC-IMM unit within the project were initially limited to supply fabrication technology for superconducting devices, such as state-of-the-art micro-SQUIDs and superconducting circuits and switches for magnetic qbit preparation and characterisation. Evolution of the project forced the activity of this unit to focus on extremely small micro-SQUID development, trying to extend the device sensitivity beyond state-of-the-art and to be able to measure single spin clusters with S=10. Interest on this extremely difficult to achieve objective with CSIC-IMM experience and technology faded out as measurement attempts at NPL and contacting difficulties resulted in blown-up or short-circuit devices at low temperatures.
The experience gained through these developments is of great value for our scientific community and society. First, because this project has directly promoted the establishment of the first operative e-beam nanofabrication laboratory in our country and helped training technical personnel in a field previously absent. Second , because this know-how has effectively spread to other two labs in Spain (UAM-CSIC and U Zaragoza) using e-beam lithography at present. Third, because the competitiveness of the fabrication technology at IMM has been enhanced to the point in which, for the first time, it is possible to start competing in high tech projects for the ESA (Superconducting Bolometers for X-Ray Detection and Spectrometry). Synergies with other relevant projects within the group such as IST-NANOMAT and recently SANDIE NoE have been also very positive, as extending the range of applications to the III-V compound nanostructures.
The work performed has been mostly involved with the synthesis and characterisation of new Mn12 clusters with interesting optically-active ligands to find synergistic effects. Two complementary techniques, magnetic susceptibility and magnetic circular dichroism (MCD), were used and it was shown, for the first time, that the magnetic and optical responses are essentially identical. The development of new methods for measurement magnetic and spin properties of isolated molecules is important when the detection limits of traditional SQUID instruments are reached.
Moreover, we have set up a new crystallisation technique based on the use of supercritical fluids. Although their use is well-known for instance, for the controlled crystallisation and delivery of pharmaceutical drugs, this was the first time that this technique was used to obtain nanocrystals of molecular magnets. This fact is not only important because the nanocrystals obtained in this way exhibit a small dispersion range, but also because their size and shape can be controlled at will, influencing their properties when compared with the materials obtained by traditional crystallisation techniques.
The NPL group obtained several SQUID from various sources, both from within the consortium and from external contacts which were made during the course of the year and through its co-funding UK DTI activities. A SQUID sensitivity of the order of just 40 spin/Hz1/2 at 4.2°K was proven by this group. The results were disseminated to other members of the consortium to allow them to improve and enhance their SQUID fabrication. Micro-SQUID fabrication technology was also successfully developed at CSIC-IMM along the second year and operative submicron dimensions devices were processed and distributed for measurements to the partners. A novel approach towards quantum limited spin detection of nanomagnets or magnetic clusters was initiated at CSIC-IMM. Novel nanomechanical devices in GaAs technology promised extraordinary sensitivity and high frequency resonance detection capabilities at low temperatures.
Measurements of the magnetisation of single crystals of Mn12 as a function of temperature and magnetic field were made by NPL using a SQUID susceptometer at Oxford University, UK. Also NPL set up a measurement method to detect the microwave radiation emanating from single crystals using novel high temperature superconductor Josephson junctions.
In addition, two different types of experiments looking for radiation emission were performed by UB. In the first type of experiments, the radiation was directly detected by using bolometers and transporting the radiation using waveguides. From these experiments the power emission was correlated to the different parameters governing the emission of superradiance in general.
In the second type of experiments, the tunnelling transition for fast sweep rate was found to be in agreement with the emission of superradiance and not longer governed by the Landau Zener law NPL masked magnetisation curves on several samples of Mn12 single crystals supplied by CSIC. These measurements were performed using a SQUID magnetometer/ susceptometer system at Oxford University allowing to form a close working relationship with their group.
In collaboration with Chalmers University (Sweden), NPL also incorporated novel high temperature superconductor Josephson junction detectors. This link to a foremost European institute is proving very beneficial to both parties. Combining the measurement expertise at NPL with the fabrication facilities at Chalmers puts Europe in a very strong position to lead future work in the THz detection and source areas which may well have a very important impact on future quality of life.
We have delivered theoretical results on various areas of quantum information processing (QIP)implementation. These results are published (or in the process) and have impact for future experiments on magnetic qubits, but also wider impact for solid state QIP devices, as described below:
i) Study of the continuous measurement of solid state qubits. This has yielded understanding of the fundamental decoherence effects induced by various measurement schemes into the qubits they are measuring has been reached. This is relevant for magnetic qubits probed by SQUIDs, but has rather wider impact in being applicable to other solid state qubit systems and devices.
ii) Understanding of the operation of a DC SQUID or large current-biased Josephson junction operating as a qubit measurement system, in particular the knock-on effects to the qubit of the intrinsic irreversibility of a current fed to the measurement apparatus. This has impact for the use of such SQUID systems in any quantum measurement scenario.
iii) Understanding of the behaviour of a microwave-driven solid state qubit subject to decoherence - the effect of the Rabi oscillation signal on the measurement apparatus. This has impact for any solid state system driven with an external classical oscillatory field. In particular, various useful parameters concerning the qubit and its environment can be extracted from this measurement scenario.
iv) Study of the readout process (continuous observation and single shot projection) for matter qubits. Conventional approaches to QIP assume the existence of single shot projective readout for qubits, so this is the ultimate aim for any qubit system.
v) Novel proposals for hybrid matter/optical QIP. In the short term it is important to simply get solid state qubits working. However, taking a longer term perspective and thinking about actual QIP devices and technology, it is likely that communication will come into play. Hybrid matter/optical QIP systems are likely to play a very important role here.
vi) Study of the probing of magnetic qubits using external thermal noise. This is a useful investigative technique for plotting out magnetic susceptibilities of qubits. The groundwork was done many years ago by one of the participants for SQUID systems - this work can be readily adapted for nanomagnets.
vii) New approaches to detection and quantum gates based on the interactions between magnetic qubits and a common microwave bus mode. This work can be applied to any solid state qubits that couple to microwaves. Further ideas continue to emerge in this area.
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